Spectroscopic measurements of quantities of interest (e.g., gas concentration) often rely on spectroscopic measurements at several frequencies. One common example is determination of gas concentration from a measured spectral absorption line of the gas. Here the integrated absorption over the spectral line can be used to determine the gas concentration.
Cavity enhanced optical spectroscopy makes use of an optical resonator to improve instrument performance. Cavity ring-down spectroscopy (CRDS) is one such method, where cavity energy decay times (i.e., cavity ring-down times) are measured in order to determine the absorption provided by a sample. In such instruments, it is important to consider the effect of the cavity modes on spectral absorption data. For example, in CRDS two operating modes have been considered in the art.
In the first CRDS operating mode, the optical source frequency is held at a nominally fixed value and the length of the cavity is varied such that cavity modes sweep through the source frequency, thereby generating ring-down events at the fixed source frequency. This operating mode can be referred to as a swept cavity mode. Spectral data in the swept cavity mode is obtained by tuning the source to the desired frequencies and sweeping the cavity length long enough at each of these source frequencies until sufficient data has been collected.
In the second CRDS operating mode, the cavity length is held at a nominally fixed value and the frequency of the source is varied such that the source frequency sweeps through one or more of the cavity mode frequencies, thereby generating ring-down events at the cavity mode frequencies. This operating mode can be referred to as a swept source mode. A single source frequency sweep in this mode provides absorption data points at frequencies that are spaced by the free spectral range (FSR) of the cavity. Measures to increase resolution in this mode have been employed. For example, the cavity length can be changed between successive source frequency sweeps such that frequency resolution is improved.
In either case, the resulting raw data for this kind of measurement generally has data points that are measurements at various frequencies (e.g., (ν, α(ν)) pairs, where ν is frequency and α(ν) is absorption at that frequency). Errors in the frequency ν of these data points can undesirably reduce the accuracy of the final determination of gas concentration.
It would be an advance in the art to provide spectroscopic methods that are less reliant on accurate frequency values in absorption data.